It is known to perform Data Dependent Acquisitions (“DDA”) using a mass spectrometer wherein an initial survey scan of parent ions is performed. Once an initial survey scan has been performed, parent ions of interest are determined and are then isolated or selected by a quadrupole mass filter. The parent ions of interest are isolated or selected by arranging for the mass filter to transmit just the specific parent ions of interest on the basis of the mass to charge ratio of the parent ions. The parent ions of interest may then be fragmented and the resulting fragment ions may then be mass analysed. Mass analysis of the resulting fragment ions enables the parent ions of interest to be identified or the identity of the parent ions of interest to be confirmed.
However, this known approach suffers from the problem of having a relatively low duty cycle since a plurality of species of parent ions will arrive at the quadrupole mass filter at substantially the same time but only a single species of parent ions of interest will be onwardly transmitted by the mass filter. The other parent ions of interest will be attenuated by the mass filter.
The known approach also exhibits insufficient selectivity of the isolation step when processing complex mixtures. For example, two different species of parent ions may have substantially the same mass to charge ratio but the quadrupole mass filter will transmit both species of parent ions since the mass filter is unable to distinguish between the two different species of parent ions.
An improvement to the known approach is described in WO 2013/140132 (Micromass) which discloses an approach wherein an initial multi-dimensional survey scan is performed comprising separating parent ions according to a first physico-chemical property (e.g. ion mobility) and then separating the parent ions according to a second physico-chemical property (e.g. mass to charge ratio). A plurality of parent ions of interest are then determined from the initial multi-dimensional survey scan. The plurality of parent ions of interest are then sequentially selected based upon the first and second physico-chemical properties during a single cycle of separation. The parent ions of interest may then be fragmented and corresponding fragment ions may then be mass analysed.
The multi-dimensional separation of the ions results in improved specificity of the precursor or parent ions (or resulting fragment ions), improved isolation and significantly improved duty cycle compared the known approach as described above.
However, during the single acquisition or cycle of separation after the initial multi-dimensional survey scan, the system is operated continuously in a selective (narrow-band) mode of operation (at sequential different values of the second physico-chemical property) so that for regions of the data where no parent ions of interest are present, ions will be lost to the system and no useful data will be acquired.
It is therefore desired to provide an improved mass spectrometer and an improved method of mass spectrometry.